1. Field of the Invention
This invention relates generally to the field of DC-to-DC converters and more particularly, to a DC-to-DC converter that uses an H-bridge driver as the switching device for alternately energizing two inductors wound about a common magnetic core.
2. Description of the Related Art
Electronic systems typically draw power from an AC line source. A received AC voltage is then converted to a DC voltage by an AC-to-DC converter for powering system components requiring DC voltages. Generally, many components within an electronic system require different magnitudes of DC voltages, for example, a flat panel display might require 36 volts while semiconductor integrated circuits might require 5 volts. The different DC voltages can be provided by DC-to-DC converters. DC-to-DC converters generally rely upon the storage characteristics of inductors and capacitors while alternately switching the applied DC voltage between the storage network and a ground potential. Thus, a converted DC voltage is generally a sawtooth waveform, first charging towards the applied DC voltage, then discharging towards ground. The sawtooth waveform is then filtered to provide a substantially ripple free DC voltage.
Typical DC-to-DC converters include SEPIC, Buck, Boost, and Flyback converters. A SEPIC converter 10 is shown in FIG. 1. In the SEPIC converter 10, a DC voltage. V.sub.IN is applied at an input terminal 1, and a converted DC voltage. V.sub.OUT appears at an output terminal 2. As shown in FIG. 1, the SEPIC converter 10 requires at least four inductors and several parallel capacitors 4 and 7 which directly contribute to size and cost. A significant drawback associated with the SEPIC converter 10 is a direct result of using power Schottky diodes 9. The power Schottky diodes 9 typically have a forward voltage of approximately 0.5 to 0.6 volts. This forward voltage substantially adversely affects a conversion efficiency of the SEPIC converter 10 since all output current passes through the power Schottky diodes 9. As power requirements increase, the power loss due to the power Schottky diodes 9 increases accordingly. Additionally, the output current at node A is a sawtooth waveform requiring adequate filtering to remove ripple from a voltage at node A. Inductor 11 and capacitor 13 filter the voltage to provide the converted DC voltage, V.sub.OUT at the output terminal 2. The magnitude of V.sub.OUT is determined primarily by the magnitude of V.sub.IN and the frequency at which switch 6 operates (i.e. the time switch 6 remains open relative to the time switch 6 is closed).
Demand for improved power supplies requires decreasing cost and size while improving performance. Performance, in part, can be measured by the conversion efficiency of a DC-to-DC converter, that is, how much power is lost when converting voltages. The conversion efficiency is the ratio of the output current multiplied by the output voltage and divided by the input power. Another measure of performance is determined by the amount of ripple in the converted voltage, which in turn determines the extent filtering will be required. Good performance, then, can be improved by eliminating power consuming devices (switching elements and diodes) and typically requires using large inductors and capacitors both for developing the sawtooth waveform and for adequately filtering that waveform. Larger storage components necessarily increase cost and size. Performance can also be improved by increasing the frequency of the switching element. Switching elements, however, are physically limited to the switching speeds of currently available switching devices. The problems associated with DC-to-DC converter design are further exacerbated when larger output currents are required since larger (and hence slower) switching devices are required, the power losses in conversion are potentially large, and still larger inductors are required.
Thus what is needed is a DC-to-DC converter that provides a converted voltage requiring minimal filtering while maintaining a high conversion efficiency by eliminating power Schottky diodes and increasing the switching frequency with available switching devices for high current outputs.